This invention generally relates to systems and methods for detecting and determining a line-of-bearing to transient radiofrequency (RF) or acoustic signals received by an antenna or transducer having multi-beamforming capability. The invention will be described as applying to RF signals received by antenna arrays, but can just as easily be described, mutatis mutandis, as applying to acoustic signals received by transducer arrays as well.
A typical system that would be employed to detect and determine a line-of-bearing to transient RF signals would use either a fast-scanning single-beam antenna to cover a large geographical area or multibeam antenna, such as a digital beamforming antenna, that can simultaneously cover the same geographical area as a scanning antenna. These systems would typically use a linear, low-pass filter at the output of each beam to determine the noise level of the environment, from which a detection threshold is derived. When an incoming signal exceeds the detection threshold, a signal detection is declared.
Single-beam scanning systems are inadequate for detection of short-duration transient signals simply because the antenna beam may not be pointed at the source of the transient signal during its transmission. Hence, the system would miss the transient signal entirely. Systems which scan faster in order to cover a desired geographical region more quickly, and not miss short-duration transient signals, result in the antenna beam only partially capturing longer-duration transient signals as the antenna beam scans by them. Widening the single-beam partially solves this problem, but results in a reduction in the accuracy of a line-of-bearing measurement to the source of the transient signal. As a result of these difficulties, a multibeam system is required for an effective solution.
Existing multibeam solutions typically apply linear low-pass filters to the output of each beam of an antenna to determine the average RF noise level of the environment, from which a detection threshold is derived. When a signal exceeds this threshold, detection of a transient signal is declared. It is desired that the detection threshold remain above the background noise level and above the level of long-duration continuous-wave (CW) signals so that noise and long-duration signals are not reported as transient signals. It is also desirable that this detection threshold remain below the level of transient signals so that these transient signals will be detected when they exceed the detection threshold. Hence, it is desired that the detection threshold remain unaffected by transient signals. This does not happen with linear filters. Due to the nature of the linear low-pass filter used to derive the detection threshold described above, the threshold is based upon the average signal power, regardless of whether the signal is a CW signal or a transient signal. Although one can reduce the effect that transient signals have on a linear, low-pass filters output by lowering the filters cutoff frequency, doing so introduces the problem of long settling times required when setting the threshold level.
U.S. Pat. No. 7,459,962 discloses a system and a method for detecting transients within a signal having a varying noise floor and long-duration CW signals without the transients themselves influencing the setting of the detection threshold. This is accomplished using nonlinear, order statistic filters applied to power spectral estimates of ambient signals.
There is a need for a system and a method for detecting and determining the line-of-bearing to transient signals received by a multi-beamforming antenna which do not suffer from the aforementioned disadvantages.